In recent years, optical transmission systems that transmit an optical signal faster than 40 Gbit/s at a single frequency have been studied and developed. In the ultra high-speed optical transmission system, an optical signal-to-noise ratio (OSNR) is degraded, and waveform distortion is caused due to chromatic dispersion of an optical fiber and other reasons. As a technology to solve these problems, a digital coherent receiver technology has been drawing attention.
A digital coherent optical receiver includes a front-end circuit, an A/D converter, and a digital signal processing device. The front-end circuit receives an optical signal by utilizing a local optical source and generates analog electrical signals representing the optical signal. The A/D converter converts the analog electrical signal output from the front-end circuit into digital signals. The digital signal processing device demodulates the optical signal by performing digital operations on the digital signals.
The digital signal processing device includes, for example, a waveform distortion compensator that mainly compensates for chromatic dispersion and an adaptive equalizer that mainly compensates for waveform distortion caused by polarization fluctuation. In this configuration, when the chromatic dispersion is sufficiently compensated in the waveform distortion compensator, the adaptive equalizer has to compensate hardly any chromatic dispersion. As a result, the circuit size of the adaptive equalizer may be made small. Consequently, the adaptive equalizer can eliminate delay caused by its circuit size, and can also perform high-speed feedback responding to fast polarization fluctuations. For that reason, it is desirable that the chromatic dispersion is sufficiently compensated in the waveform distortion compensator.
The chromatic dispersion may be estimated and compensated by the following methods.    (1) The chromatic dispersion is estimated based on the distance of a transmission path and a characteristic of an optical fiber. In this method, however, there is a large margin of error in the estimation. Therefore the chromatic dispersion may not be sufficiently compensated.    (2) In the error correction procedure for demodulated signals, the amount of chromatic dispersion compensation is determined so that the number of error corrections is minimized. However, since this method is performed after all operations of the optical receiver (including frame synchronization) are terminated, the estimation time becomes longer. For that reason, transmission systems that have their optical paths switched at the time of failure, for example, may experience a delay in system recovery.    (3) The amount of chromatic dispersion compensation is estimated by using clock signals recovered by an analog clock recovery circuit. However, since residual dispersion is greater in analog regions, it is difficult to accurately estimate the amount of chromatic dispersion compensation.
It should be noted that methods for estimating the amount of chromatic dispersion compensation are disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2002-208892, Japanese Patent Application Laid-Open Publication No. 2004-236097, Japanese Patent Application Laid-Open Publication No. 2008-58610, and Japanese Patent Application Laid-Open Publication No. 2007-60583.
As explained above, according to the conventional arts, it has been difficult to accurately estimate chromatic dispersion in a short time in a digital coherent optical receiver. For that reason, at the time of, for example, switching optical paths that transmit optical signals, it has taken a long time to make settings for compensating chromatic dispersion of a new optical path. In other words, the time required for recovery has been long when a failure occurred in an optical transmission path.